Nuclear magnetic logging apparatus coil configuration

ABSTRACT

An illustrative embodiment of the invention selectively shortens the relaxation time of the signal from precessing protons observed with a nuclear magnetic logging tool. The region affected is located immediately adjacent to the formation side of the polarizing-receiving coil, thereby reducing the adverse response resulting from failure of the coil to be positioned closely against the face of the formation. This improvement is obtained through a vertical strip that contains a ferromagnetic material that is located inside of the tool housing generally coextensive with the coils.

United States Patent Willie C. Culpepper 72 Inventor 3,179,878 4/1965Schvvede 324/05 Houston, Tex. 3,289,072 1 l/ 1966 .Schuster 324/0.5 [21]p 845786 Primary ExaminerMichael .l. Lynch [22] Filed July 29, 1969 YAttorneys-William R. Sherman, Donald H. Fidler, Stewart F. [45] PatentedJuly 13, 1971 M d1 h P n [73] Assignee Schlumberger TechnologyCorporation core an o n New York, N.Y.

[54] NUCLEAR MAGNETIC LOGGING APPARATUS ABSTRACT: An illustrativeembodiment of the invention COIL CONFIGURATION selectively shortens therelaxation time of the signal from 2 Claims, 1 Drawing Fig.

y precessmg protons observed with a nuclear magnetic logging [52]U.S.Cl. 324/0-5 tool, The region affe ted is located immediatelyadjacent to l f l 27/78 the formation side of the polarizing-receivingcoil, thereby 1 Field of Search reducing the adverse response resultingfrom failure of the coil to be positioned closely against the face ofthe formation. This [56] Reknnm cited improvement is obtained through avertical strip that contains UNITED STATES PATENTS a ferromagneticmaterial that is located inside of the tool 2,770,773 1 H1956 Cooley324/8 housing generally coextensive with the coils.

ll-w lO PATENTEU JUL 1 3mm INVENTOR. William C. Culpepper BY fi ATTORNEYNUCLEAR MAGNETIC LOGGING APPARATUS COIL CONFIGURATION BACKGROUND OF THEINVENTION an earth formation traversed by-a borehole, tools have been.proposed to measure the formations free fluid index. Typically,according to these suggestions, one or more large induction coils areenclosed within a fiber glass housing and drawn through a borehole. Asdescribed in more complete detail in John H. Baker, Jr. Pat. applicationSer. No. 537,475 filed Mar. 25, 1966, now U.S. Pat. No. 3,483,465,granted to J. H. Baker,

Jr. on Dec. 9, 1969, for NuclearMagnetism Logging System Utilizing anOscillated Polarizing Field a high intensity direct,

current is applied to the tool coils in order to produce a mag neticfieldthat polarizes in one direction some of the protons in.theformation liquids, the mudcake and the borehole fluid. A netmagnetization is produced by the polarization of the protons. After asuitable period of polarization, the applied field is quickly reducedleaving the polarized proton to precess about the earth's magneticfield. This precession produces an oscillating magnetic fieldwhichinduces an electrical signal in the coil. T he'desired electricalsignal from the protons in the formation liquid cannot be observedimmediately after termination of the polarizing current because ofswitching transients andsignals from protons in the borehole fluid. The

"latter is made'to decrease very rapidly by addition of a small .amountof magnetite to the borehole fluid. The magnetic remanence of the smallmagnetite particles distorts the earth s -magnetic field in the boreholeand causes the protons in the borehole fluidto rapidly dephase. The moreslowly decaying signal from the protons in theformation fluid canthereby be observed by measuring the signal only after a delay of to 40milliseconds after termination of the polarizing current.

The free fluid volume indication is related to the initial amplitude ofthe long component induced precession signal.

Through extrapolation, the initial amplitude of the induced precessionsignal can be determined-with reasonable accura- Because of the physicalnature of the borehole, the coil portion of the tool cannot alwaysbe;positioned immediately adjacent to the formation. Whenthis occurs, areduced volume i of formation fluid is sampled andvthe derived freefluid measurement is correspondingly in error. The errors resulting fromthis condition can be reduced if a means-is employed to make the toolinsensitive to response from protons located within one-half inch fromthe face of the coil. A thick shell of .fiber glass reinforced withplastic would achieve this purpose,

but the resulting increase intool diameter limits the minimum ]holediameter that can be traversed.

SUMMARY OF THE INVENTION In accordance with an'exemplary embodiment ofthe invention, the decay time of the signal adjacent to the logging toolis reduced 'by interposing a small lengthwise strip of ferromagneticmaterial between the coils and the borehole wall.

glue. A typical magnetite concentration of approximately 87.8Xl0 gramsper square inch saturating a one-half inch wide strip of fiber glasscloth that is adjacent to and coextensive with the length of the coilassembly and glued to the inner surface of the housing has been quitesatisfactory.

For a better understanding of the present invention, together withotherand further objects thereof, reference rs had to the followingdescription taken in connection with the accompanying drawings, thescope of the invention being pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE of the drawing shows,in transverse section, an exemplary embodiment of the invention in atypical bore hole logging tool.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 1 In the embodiment of theinvention illustrated in the drawing, a logging instrument 10 issuspended for transport through a borehole (not shown) by amulticonductor cable (also not shown). Preferably, the logginginstrument 10 is urged against the borehole wall by a bowspring 11.

Within the instrument l0, conductors l2 and 13 lead from polarizing coilmeans 14 to signal processing circuits that produce free fluid relatedsignals (not shown) described, for example, in connection with theaforementioned Baker, Jr. patent application.

Turning once more to the logging instrument 10, the coil assembly 14comprises a winding 15 that is vertically elongated for better fieldconcentration. A typical height to width ratio for the winding 15 is,for example, about 3 to l.

Theouter surface of the winding 15 is covered by means of nonmagneticcores 16 and 17 that are secured to a nonmagnetic backing plate 20. Thebacking plate 20 supports the entire coil means 14 and is secured to aninstrument'housing 22 with nonmagnetic bolts or fasteners. The boltsextend through the cores l6 and I7 and the backing plate 20. Preferably,the

cores l6 and 17, the backing plate 20 and the protective outer coveringor housing 22, can be made of fiber glass cloth layers laminated with anepoxy resin. As an illustrative alternative, a

nonmagnetic stainless steel also provides a suitable nonmagneticstructural material. To provide a coil assembly that is watertight atthe high pressures usually encountered in boreholes, the assembly 14 iscovered with a layer of sealing material 21, such as rubber or neoprene.

In accordance with the invention, a lengthwise strip of ferromagneticmaterial 23, preferably magnetite, is applied to the logging instrument10. It has been found, for example, that a one-half inch wide piece offiber glass cloth as shown in the drawing, impregnated with 87.8 l0-grams of magnetite per square inch, if glued to the common surfaces ofthe proec iv g srj sl anqths Sealing ma r a 21.5. 1952 1 fiel11.

eliminates the effect of the proton signal from the portion of theformation that is adjacent to the sealing material. Cloth widths up totwo inches and less than one-quarter inch also have produced acceptableresults.

As shown, the ferromagnetic stripis glued between the fiber glass covershell and the coil winding 15. The magnetite strip, moreover, ought tobe coextensive with the vertical extent of the winding 15. Epoxy resinhas been found to provide a suitable glue for both impregnating thecloth piece with magentite and for securing the doped" cloth to theinstrument.

While there have been described what are at present considered to bepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madewithout departing from the invention and it is, therefore, intended tocover all such changes and modifications as fall within the true spiritand scope of the invention.

What I claim is:

l. A borehole logging tool comprising a nonmagnetic housing forengagement with the borehole wall, coil means within said housing forproducing a current in response to the magnetic field generated byprotons precessing in the earth formation traversed by the borehole, astrip of fiber glass cloth about one-quarter of an inch wide coextensivewith the longitudinal dimension of said coil means interposed betweensaid coil -means and said nonmagnetic housing and impregnated withmagnetite for suppressing the effect of precession signals netite stripis impregnated with about 87.8 X 10 magnetite per square inch.

grams of

1. A borehole logging tool comprising a nonmagnetic housing forengagement with the borehole wall, coil means within said housing forproducing a current in response to the magnetic field generated byprotons precessing in the earth formation traversed by the borehole, astrip of fiber glass cloth about one-quarter of an inch wide coextensivewith the longitudinal dimension of said coil means interposed betweensaid coil means and said nonmagnetic housing and impregnated withmagnetite for suppressing the effect of precession signals generated inthe portion of the earth formation adjacent to the borehole.
 2. Alogging tool according to claim 1 wherein said magnetite strip isimpregnated with about 87.8 X 10 3 grams of magnetite per square inch.